Regarding the techniques of canceling echoes leaking from the transmitter side to the receiver side on the 4-line side of a 2-4 wire hybrid transformer circuit, there is known an echo canceler described in “Adaptive Signal Processing”, 1985, Prentice-Hall Inc., USA (reference 1).
The echo canceler employs an adaptive filter having a number of tap coefficients equal to or larger than an impulse response length of an echo path to produce a pseudo echo (echo replica) corresponding to a transmission signal, thereby operating to suppress an echo leaking from a transmission circuit to a reception circuit on the four-line side of the 2-4 wire hybrid transformer circuit.
In this event, each tap coefficient of the adaptive filter is modified by correlating the transmission signal to an error signal which is calculated by subtracting the echo replica from a mixed signal comprising a mixture of the echo and a received signal.
As a typical coefficient adaptation algorithm for such an adaptive filter, there are known an LMS algorithm described in the aforementioned Reference 1, and a normalizing LMS (NLMS) algorithm described in “Adaptive Filters,” 1985, Kulwer Academic Publishers, USA (Reference 2).
On an actual communication line, a plurality of subscriber lines are multiplexed to form multiplexed lines for further improving efficiency of the transmission capacity. In such a case, echo cancelers for canceling echoes in a 2-4 wire hybrid transformer circuit are equipped in a multiplexer as many as the number of multiplexed lines. A design for permitting a reduction in the total amount of operations in such an echo canceler for multiplexed lines is described in Proceedings of Symposium on Digital Signal Processing of the Institute of Electronics, Information and Communication Engineers of Japan, pp. 671-676, November 1999 (Reference 3). FIG. 1 illustrates the configuration of multiplexed echo cancelers described in Reference 3 when the number of multiplexes is three.
On a first line, a transmission signal fed to transmission signal input terminal 1 is sent to a transmission path from transmission signal output terminal 2, and sent to a 2-line side in 2-4 wire hybrid transformer circuit 3, wherein a portion of the transmission signal leaks into a reception side as an echo due to mismatch in impedance and the like.
This echo is fed from received signal input terminal 4, and is supplied to subtractor 5. On the other hand, adaptive filter 86 receives input signal 700 supplied to transmission signal input terminal 1, and produces echo replica 701 through a convolution with a coefficient value of adaptive filter 86 modified on the basis of error signal 702, which is an output of subtractor 5. Subtractor 5 subtracts echo replica 701 delivered from adaptive filter 86 from the echo leaking into the reception side, and transfers the result of the subtraction thus calculated to received signal output terminal 6. The result of the subtraction is also fed back simultaneously to adaptive filter 86 as error signal 702 for updating the coefficients.
Control circuit 79 receives step sizes 601, 603, 605 from adaptive filters 86, 87, 88, respectively, and evaluates them. Since the step sizes increase from initial values and decreases as the coefficients are updated, they represent to what extent adaptive filters 86, 87, 88 converge. Control circuit 79 supplies coefficient update control signals 602, 604, 606 at predefined time intervals corresponding to adaptive filters 86, 87, 88. Coefficient update control signals 602, 604, 606 are information which determines how many times the respective adaptive filters update coefficients in the predetermined time interval, and are determined depending on the result of the evaluation on the step sizes. Since the configuration and operation of the echo cancelers in a second and a third line in FIG. 1 are completely the same as those of the first line, description thereon is omitted.
Since the conventional echo canceler for multiplexed lines does not need the number of operations which increases in proportion to the number of multiplexed lines, the amount of operations can be reduced.
However, when an input signal is an audio signal, an allocated number of times the coefficients are updated may not be used but wasted in some cases. This is because the coefficients are not updated when an input signal has a small amplitude, so that even if coefficient updates are allocated to an adaptive filter associated with such a line, the adaptive filter does not substantially update the coefficients. In other words, the number of operations associated with the allocated coefficient updates may include waste.